VIRGOHI21

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VIRGOHI 21
Observation data (
km/s[1]
Distance50 Mly[2]
Apparent magnitude (V)< 27
Notable featuresNo stars

VIRGOHI21 is an extended region of neutral

NGC 4254
.

Observational properties

VIRGOHI21 was detected through

Doppler shift of the emissions, astronomers determined that the gas has a high velocity-profile width; that is, different parts of the cloud are moving at high speed relative to other parts. Follow-up Hubble Space Telescope
deep observations of the region have detected very few stars (a few hundred).

Dark galaxy interpretation

If the high velocity-profile width of VIRGOHI21 is interpreted as

NGC 4254
and the bridge of neutral hydrogen extending between the two entities.

Under this interpretation, VIRGOHI21 would be the first discovery of the dark galaxies anticipated by simulations of dark-matter theories.[3] Although other dark-galaxy candidates have previously been observed, follow-up observations indicated that these were either very faint ordinary galaxies or tidal tails.[4] VIRGOHI21 is considered the best current candidate for a dark galaxy.

Tidal tail interpretation

Sensitive maps covering a much wider area, obtained at

NGC 4192), which is now somewhat distant.[7] Other debris tails of this magnitude have been found to be common features in the Virgo cluster, where the high density of galaxies makes interactions frequent.[8][9]
These results suggest that VIRGOHI21 is not an unusual object, given its location at the edge of the densest region of the Virgo cluster.

The original paper describing VIRGOHI21 as a dark galaxy provides several objections to the tidal-tail interpretation: that high-velocity interactions do not generally produce significant tails, that the high velocity needed is out-of-place in this part of the Virgo cluster and that the observed velocity profile is opposite from that expected in a tidal tail. In addition, according to Robert Minchin of the Arecibo Observatory, "If the hydrogen in VIRGOHI21 had been pulled out of a nearby galaxy, the same interaction should have pulled out stars as well".[10] Proponents of the tidal-tail interpretation counter these objections with simulations and argue that the apparently inverted velocity profile is due to the orientation of the tail with respect to Earth-based observers.

Although as of 2009 the nature of VIRGOHI21 remains a contentious issue, its identification as a dark galaxy seems much less certain now than immediately after its discovery.

See also

References

  1. ^ a b c "NASA/IPAC Extragalactic Database". Results for VIRGOHI 21. Retrieved 2006-11-19.
  2. ^ a b "New evidence for a Dark Matter Galaxy". Particle Physics and Astronomy Research Council (PPARC). Archived from the original on 2007-03-10. Retrieved 2008-02-26.
  3. S2CID 15639006
    .
  4. S2CID 1097040
    .
  5. S2CID 12717336
    .
  6. S2CID 12930657
    .
  7. S2CID 15348867
    .
  8. S2CID 16625759
    .
  9. doi:10.1086/521100
    .
  10. ^ Stuart Clark. "Dark galaxy' continues to puzzle astronomers". New Scientist. Retrieved 2008-02-26.

External links


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